Conventionally, the panel structure of car body members such as for automobiles uses a closed sectional structure through spaces in combination with an outer panel (hereafter simply referred to as an outer) and an inner panel (hereafter simply referred to as an inner).
The panel structures for a car hood, a roof, doors, and the like especially use mechanical, soldering, and adhesive means such as resins to combine the outer and the inner that is provided at one side of the outer toward the bottom of the car body to reinforce the outer.
The inner or both the inner and the outer for these car body panel structures are becoming using highly rigid and moldable aluminum alloy plates such as AA or JIS standard compliant 3000, 5000, 6000, and 7000 series for weight saving in addition to or instead of the conventionally used steel. Hereafter, aluminum is simply represented as Al.
Recently, car panel structures including the Al alloy plates need to be highly rigid as well as thin and light-weight. The member characteristics need to excel in the bending rigidity, the torsional rigidity, and tension rigidity (dent resistance).
Conventionally, car body hood inners are available in the beam type and the cone type. The beam-type inner provides each panel with a trim section for weight saving. The cone-type inner has no trim section on the basis of the closed sectional structure. Relatively large convex sections (protrusions) called cones are arranged on the cone-type inner at a regular interval. Each cone has a trapezoidal sectional view. With respect to the tension rigidity and the bending rigidity, a hood structure using this inner (cone-type hood structure) is equivalent to a structure using the beam-type inner (beam-type hood structure) in accordance with the rigidity design of the hood. On the other hand, with respect to the torsional rigidity, the cone-type hood structure is approximately twice as rigid as the beam-type hood structure. Recently, special attention is paid to the cone-type hood structure.
Recently, from the viewpoint of protecting pedestrians, hood design requirements tend to consider the safety against impact to a pedestrian's head. Concerning the beam-type hood structure, there are several disclosures (JP-A Nos. 165120/1995, 285466/95, and 139338/93). In addition, the EEVC (European Enhanced VeHICle-Safety Committee) specifies an HIC value of 1000 or less as a hood condition with respect to the impact resistance to adult and child heads (described in EEVC Working Group 17 Report, Improved test Methods to evaluate pedestrian protection afforded by passenger cars, December 1998).
However, the prior art is accompanied by the following problems.
(1) [Problem 1] Increasing the Tension Rigidity
There may occur cases where the conventional cone-type and beam-type inners cannot satisfy the demand for increased rigidity when they are thinned and made to be lightweight.
FIG. 13(a) is a longitudinal sectional view of an inner. FIG. 13(b) is a plan view of the inner. As shown in these figures, there are arranged many conic convex sections (protrusions) 14 at a regular interval on the surface of a cone-type inner 13. There is formed a flat section or a concave section 16 between the convex sections 14. The reference numeral 21 represents a horseshoe bead provided at an outside periphery of the panel. The bead 21 is universally used for reinforcing the rigidity of the inner As shown in FIG. 13(a), the cone-type inner 13 is joined to an Al alloy outer 12 having a specified curvature to constitute the closed sectional structure through spaces and to be integrated into a panel structure 11. In the example of FIG. 13(a), there is provided a resin layer 15 on a flat top 14a of the convex 14 on the inner 13. The resin layer 15 is used to join the convex 14 of the inner 13 to a rear surface 12a of the outer 12. The panel periphery is hemmed (bent) to be integrated into the panel structure.
FIG. 14 is a perspective view showing an example of applying the beam-type inner to a car body hood. As shown in FIG. 14, the beam-type inner 17 comprises beams 19 appropriately crossing longitudinally, transversely, and slantwise with reference to a plane direction of the panel. The beam-type inner has a trim structure having a trimmed space section 20 between the beams 19. The beam-type inner 17 is also joined to the rear surface of an outer 18 to constitute the closed sectional structure through spaces and to be integrated into a panel structure.
The panel structure is locally reinforced by reinforcing members such as a hinge reinforcement 21 and a latch reinforcement 22 including the cone-type inner.
These cone-type hood structures are approximately twice as rigid as conventionally used general-purpose beam-type hood structures and can be assumed to be excellent in the rigid design. This is because the closed sectional structure of the cone-type hood structure excels in the rigidity against a torsional load. In addition, the cone-type hood structure has the bending rigidity equivalent to that of the beam-type hood structure. The cone-type hood structure does not necessarily provide the sufficient tension rigidity. The cone-type hood structure is requested to increase the tension rigidity.
As a result, a relatively large, thick plate must be used for the panel at the sacrifice of weight saving in order to increase the tension rigidity for the conventional cone-type inner.
Therefore, it is an object of the present invention is to provide a car body hood panel structure capable of satisfying a demand for increased rigidities such as the tension rigidity in order to take advantage of weight saving by thinning the panel on the assumption of high torsional rigidity characteristic of the conventional closed sectional structure.
(2) [Problem 2] Improving the Head Impact Resistance for Protecting Pedestrians
Generally, the head impact resistance evaluated in accordance with the following HIC (Head Injury Criteria) value with respect to Automobile Technical Handbook, Vol. 3, Test and Evaluation, 2d ed. (Society of Automotive Engineers of Japan, Inc., Jun. 15, 1992).
  HIC  =                    [                              1            /                          (                              t2                -                t1                            )                                ⁢                                    ∫              t1              t2                        ⁢                          a              ⁢                                                          ⁢                              ⅆ                t                                                    ]            2.5        ⁢          (              t2        -        t1            )      
where a is 3-axis composed acceleration (in units of G) at the head centroid, and t1 and t2 are times having the relationship of 0<t1<t2 to cause a maximum HIC value. An operation time (t2−t1) is specified to be 15 msec or less.
EEVC Working Group 17 Report specifies an HIC value of 1000 or smaller for each of impact resistances to adult and child heads as a condition attributed to the hood. In this report, the head impact test uses a head impact speed of 40 km/hr. The test specifies a weight of 4.8 kg, an external diameter of 165 mm, and an impact angle of 65 degrees for the adult head; and a weight of 2.5 kg, an external diameter of 130 mm, and an impact angle of 50 degrees for the child head.
During the head impact test, the pedestrian's head first impacts on the outer. Then, the deformation progresses to transmit a reactive force to rigid parts such as an engine in the engine room via the inner, causing an excess impact on the head. The head is subject to a first acceleration wave and a second acceleration wave. The first acceleration wave is mainly generated by impact against the outer approximately within 5 msec from the beginning of the impact. When the inner impacts on a rigid object, the second acceleration wave is generated approximately 5 msec or later from the beginning of the impact. The elastic rigidity of the outer mainly determines the magnitude of the first acceleration wave. The elastoplastic rigidity of the inner mainly determines the magnitude of the second acceleration wave. Deformation energies for the outer and the inner absorb the kinetic energy at the head. If the head's movement distance exceeds a clearance between the outer and a rigid object such as the engine, the head is directly subject to a reactive force from the rigid object. Consequently, the head is subject to a fatal damage equivalent to an excess impact greatly exceeding the maximum HIC value of 1000.
(3) [Problem 2-1] Capable of Decreasing the HIC Value Despite a Small Head Movement Distance
According as a clearance is increased between the outer and a rigid object such as the engine, the head's movement distance can be increased. This is advantageous to reducing the HIC value. However, the hood design inevitably is accompanied by limitations. There is a need for a hood structure capable of reducing the HIC value despite a small clearance and a short head movement distance.
More severe impact conditions are required especially for the adult's head impact than for the child's head impact. An excess clearance needs to be provided between the outer and the rigid object surface beyond the design allowance (described in EEVC Working Group 17 Report).
As another problem, it is difficult to satisfy the HIC value of 1000 for both children and adults with different impact characteristics along the line WAD1500 that provides a possibility of head impacts both for children and adults. The line WAD1500 indicates a 1500 mm distance along the border line from the ground surface at the car body front to the hood impact position. More particularly, the line WAD1500 for a large sedan' hood is located immediately above the engine so that just a small clearance is left between the outer and the rigid object surface, causing a demand for effective countermeasures. (described in EEVC Working Group 17 Report)
(4) [Problem2-2] Uniform HIC Value Independent of Impact Portions
With respect to head impact positions, a large HIC value results immediately above the frame for the beam-type hood structure or at the cone vertex for the cone-type hood structure. This is because these portions provide high local rigidity, cause small deformation if impacted on a rigid object, and are subject to a high reactive force from the rigid object. From the viewpoint of safety, there has been a demand for a hood structure that can provide an approximately uniform HIC value independently of impact portions.
(5) [Problem 2-3] Applicability of Aluminum Material
The third problem to be solved is to provide an excellent head impact resistance despite the use of an aluminum material capable of weight saving as a hood material. The aluminum material is often used for light-weighting the hood. Compared to the use of the steel material, however, the use of the aluminum material is generally considered to be disadvantageous from the viewpoint of protecting pedestrians. This is mainly because the elastic modulus and the gravity of the aluminum material are approximately one third of those of the steel material. If the hood is used to absorb the kinetic energy of the head, the membrane rigidity and the weight of the aluminum hood as the panel structure are insufficient compared to those of the steel hood.
The bending rigidity of a plate material is proportional to ET3, where E is a Young's modulus and T is a plate thickness. The membrane rigidity thereof is proportional to ET. When the steel material (Young's modulus Es, plate thickness Ts, and gravity γs) is replaced by the aluminum material (Young's modulus Ea, plate thickness Ta, and gravity γa), the plate thickness is determined as follows so that the same bending rigidity results.EaTa3=EsTs3Ea/Es=1/3
Hence,Ta/Ts=31/3=1.44
A membrane rigidity ratio of the aluminum hood to the steel hood becomes:(EaTa)/EsTs=1.44/3=0.48
A weight ratio thereof becomes:(Taγa)/(Tsγs)=1.44/3=0.48
The membrane rigidity and the weight of the aluminum hood are just 0.48 times as large as those of the steel hood. As a result, when the head impacts on the hood, the head movement distance increases and the head easily impacts on a rigid object. The outer absorbs a small energy at the first acceleration wave, increasing the second acceleration wave. Accordingly, the conventional hood structure increases the HIC value, making it very difficult to satisfy limits of the HIC value.
Of course, making Ta equal to a triple of Ts provides the same membrane rigidity ratio and weight ratio as those for the steel hood. However, this causes excess costs, unpractical for the design.
In this manner, it is very difficult to use the aluminum material for the hood and limiting conditions for the head impact under this condition. Of course, if there is found an aluminum hood structure that satisfies this condition, a steel hood employing this structure can further decrease the HIC value.
As mentioned above, the following summarizes problems to be solved for the hood structure from the viewpoint of pedestrian protection as another object of the present invention.
(a) Capable of decreasing the HIC value despite a small head movement distance;
(b) Providing the approximately uniform HIC value independently of impact portions on the hood; and
(c) Capable of sufficiently decreasing the HIC value even using an aluminum hood.